This invention relates to a method and apparatus for determining the position of a coil with respect to a grid of spaced conductors and more particularly to such instruments which are relatively simple and low cost.
Various high resolution apparatus for translating a position of a movable instrument, such as a pointer or pen, into electrical signals for transmission to a local or remote utilization device are well known in the art.
U.S. Pat. No. 4,210,775, assigned to Talos Systems, Inc., pertains to a digitizer in which an instrument, such as a pen having a coil disposed about a nosepiece located immediately above the writing tip of the pen, is utilized in conjunction with a tablet comprising a grid of conductors, including a group of parallel spaced conductors oriented in an X direction and a group of parallel spaced conductors oriented in a Y direction. The pen tip is moved along a worksheet disposed on a support surface containing the grid of conductors. An oscillator applies a signal of predetermined constant frequency and amplitude to the coil. The coil is inductively coupled to the conductors of the grid, thereby inducing signals in the conductors. In accordance with electromagnetic theory, the magnitude and phase of the signals induced in the respective grid conductors depend on the location of the conductors with respect to the pen tip. The grid conductors in each group are sequentially scanned to sequentially couple the induced signals to a differential amplifier by means of multiplexing circuitry. The multiplexing circuitry selects the respective grid conductors in response to an address decoder. The address decoder is driven by a scan counter. The scan counter is incremented by a signal produced by a divider circuit driven by the oscillator. A position counter is incremented in response to the oscillator as the X and Y groups of grid conductors are sequentially scanned. A phase sensitive detector responsive to the output of the amplifier produces a positive signal having a step-like envelope as grid conductors approaching one side of the pen tip are sequentially scanned. The phase sensitive detector also produces a negative signal having a step-like envelope as conductors on the opposite side of the pen tip are sequentially scanned in a direction away from the pen tip. The signal produced by the phase sensitive detector is filtered to produce a characteristic wave shape signal having a positive peak as the scanning approaches the pen tip. The filtered signal falls steeply across the zero axis to a negative peak as the scanning passes under the pen tip, and decreases as the scanning continues in the direction away from the pen tip. The filtered signal is differentiated and compared to a predetermined threshold level to produce a stop pulse. The stop pulse is utilized to disable the position counter. At this point the contents of the position counter represent the location of the pen tip with respect to the X group of grid conductors and are loaded into an output register. The contents of the output register are then outputted to a utilization device. The scan counter and the position counter are reset and the conductors of the Y group of grid conductors are scanned in a similar manner to produce a digital position number representing location of the pen tip with respect to the Y group of conductors.
Other patents which employ multiplexers to sequentially scan the respective X and Y conductors of groups of parallel spaced conductors and assigned to Talos Systems, Inc. include U.S. Pat. Nos. 4,185,165 and 4,260,852. While all of these patents which employ multiplexers to sequentially scan the grid conductors produce desirable results in a satisfactory manner, the equipment to accomplish these results are relatively complex and costly.
Accordingly, it is an object of this invention to provide a simplified and lower cost digitizer.